powerpc/5200: improve uart baud rate calculation (reach high baud rates, better accuracy)

On the MPC5200B, make very high baud rates (e.g. 3 MBaud) accessible and
achieve a higher precision for high baud rates in general.  This is done by
selecting the appropriate prescaler (/4 or /32).  As to keep the code clean,
the getuartclk method has been dropped, and all calculations are done in a
new set_baudrate method.

Notes: only "fsl,mpc5200b-psc-uart" compatible devices benefit from these
improvements.

Tested on a custom 5200B based board, from 110 baud up to 3 MBaud, and with
both "fsl,mpc5200b-psc-uart" and "fsl,mpc5200-psc-uart" devices.
Also tested on the mpc5121ads board.

Signed-off-by: Albrecht Dreß <albrecht.dress@arcor.de>
[agust: fixed mpc5121 prescaler comment]
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>